A mechanistic study of degradation of a typical automotive clearcoat caused by bird droppings Hossein Yari, Mohsen Mohseni, Bahram Ramezanzadeh Ó ACA and OCCA 2010 Abstract This study aims at investigating the degra- dation mechanism of an automotive clearcoat caused by bird droppings. Natural bird droppings and their synthetic equivalent (pancreatin) were utilized for this purpose. The presence of highly-etched areas on the surface of coatings and structural variations of clear- coat after being attacked by these materials corre- sponded to a catalyzed hydrolytic degradation. This finding was obtained using different analytical tech- niques. Based on these studies, three possible hypoth- eses were presented, including acid catalysis, metal ion catalysis, and enzymatic catalysis. The conditions required for the occurrence of each hypothesis were discussed. It was found that acid and metal ions have a weak contribution to catalyze the hydrolysis reaction of clearcoat, whereas enzymes existing in bird drop- pings were mainly responsible for this hydrolytic degradation. Keywords Enzymatic degradation, Bird droppings, Automotive coating, Hydrolysis, Pancreatin Introduction The need to obtain proper appearance and esthetic properties of coatings in the automotive industry has led to a vast amount of research focused on the study of weathering performance of these coatings. 1–6 In this regard, humidity and sunlight are the two main factors which have been thoroughly investigated. Other envi- ronmental factors, i.e., those originated from the biological sources, have not been investigated very systemically. 7 The influence of various biological sub- stances such as bird droppings, tree gums, and insect gums on automotive coatings can have an impact on the appearance of a car body during its service life. Apart from the lack of knowledge of such a phenom- enon, with respect to the short-term or long-term effects, the current standard test methods for evaluat- ing resistance to these substances are not sufficient and cannot cover well all ranges of bio attacks. In general, Arabic gum has been considered as a universal simulated chemical to study the biological resistance of automotive coatings. 8 This, however, may not represent all biological materials to which coatings are exposed in a real condition. This article is a part of a comprehensive work, aimed to study and characterize the coating degradation caused by various biological substances. In our previ- ous papers, the effects of bird droppings on appearance and thermal-mechanical properties of coating were investigated. 9,10 Typical defects observed on the clear- coat influenced by bird droppings and pancreatin were recorded by a digital camera and are shown in Fig. 1. 11 It can be seen that these defects are stain-like white areas. The edges seem brighter and more affected compared with the interior areas. It was found that bird droppings decrease the appearance parameters of clearcoat, i.e., gloss, distinctness of image and color values, therefore affecting the esthetic properties of the coating system. 9 Thermal-mechanical studies also showed that hardness, glass transition temperature, and crosslinking density of degraded clearcoats de- crease in the presence of bird droppings. 10 Also, the influence of aging method (pre-aging or post-aging) and chemical structure of clearcoats against such bio attacks were reported. 11,12 It was observed that post- aging process which simultaneously exposes bird droppings and UV radiation to coatings degrades the clearcoat much more intensively than the pre-aging H. Yari, M. Mohseni (&), B. Ramezanzadeh Polymer Engineering and Color Technology Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran e-mail: mmohseni@aut.ac.ir J. Coat. Technol. Res., 8 (1) 83–95, 2011 DOI 10.1007/s11998-010-9273-2 83